In recent years, optical computing techniques have been developed for applications in the oil and gas industry in the form of optical sensors on downhole or surface equipment to evaluate a variety of fluid properties. In general, an optical computing device is a device configured to receive an input of electromagnetic radiation from a sample and produce an output of electromagnetic radiation from a processing element, also referred to as an optical element, wherein the output reflects the measured intensity of the electromagnetic radiation. The optical computing device may be, for example, an Integrated Computational Element (“ICE”). One type of an ICE is an optical thin film optical interference device, also known as a multivariate optical element (“MOE”).
Fundamentally, optical computing devices utilize optical elements to perform calculations, as opposed to the hardwired circuits of conventional electronic processors. When light from a light source interacts with a substance, unique physical and chemical information about the substance is encoded in the electromagnetic radiation that is reflected from, transmitted through, or radiated from the sample. Thus, the optical computing device, through use of the ICE and one or more detectors, is capable of extracting the information of one or multiple characteristics/analytes within a substance and converting that information into a detectable output signal reflecting the overall properties of a sample. Such characteristics may include, for example, the presence of certain elements, compositions, fluid phases, etc. existing within the substance.
Currently, ICEs are assessed by applying an ICE regression vector to a single set of calibration data (i.e., spectral data set) to evaluate a performance factor, for example but not limited to, a standard error of calibration (“SEC”). This procedure is performed on a set of spectral data that describes a single chemical system that contains one or more components: its target characteristic/analyte and the remaining components (including spectral interferents), usually referred to the matrix. A subset of the chemical system can be used for validation purposes to calculate the performance factor, for example, the standard error of validation; this subset represents the same chemical system and the calibration set. An illustrative ICE can be constructed as a series of alternating layers of high and low refractive index materials with associated thicknesses deposited onto an optical substrate. Such a device has an optical transmission function (T), designed by assessing a performance factor (e.g. SEC) and using a minimization function to adjust the layer thicknesses to design an ICE with an optimal performance factor (e.g. low SEC), which is thus as predictive as possible.
As stated, illustrative ICEs can be made of multiple layers of at least two materials having different complex indices of refraction. The ICEs are normally employed at fixed angle of incidence relative to the electromagnetic radiation optical path. For example, the ICE can be arranged in an optical system to allow the electromagnetic radiation to strike the ICE at a 90 degree angle, or normal to its surface. The ICE is designed to achieve the desired transmission or reflection spectroscopic profile, by considering the substrate, the complex indices of refraction of the materials, the number of layers and the thicknesses of the layers to detect a desired characteristic, such as gas/oil ratio (“GOR”), C1 through C5, etc.
There may be situations when it is desirable to fine tune the spectroscopic profile of the ICE so as to achieve enhanced resolution and/or accuracy. However, with conventional ICE configurations, once the ICE is designed and manufactured, such tuning is not possible since all the physical properties are already fixed.
Accordingly, there is a need in the art for methods by which to fine tune an existing ICE.